Process for production of aromatic polycarboxylic acids



PROCESS FOR PRODUCTION OF AROMATIC POLYCARBOXYLIC ACIDS James 0.Knobloch, Hobart, Ind., assignor to Standard Oil Company, Chicago, 11].,a corporation of Indiana No Drawing. Application May 20, 195 Serial No.660,049.

11 Claims. (Cl. 260-524) This invention relates to a processfor thepreparation of aromatic acids, and particularly to a method foroxidizing an alkylated aromatic monocarboxylic acid to produce aromaticpolycarboxylic acids. It espectially relates to a process for theoxidation of alkylated benzoic acids to produce terephthalic acid andisophthalic acid.

With the development of various new polyester resins suitable for filmand fiber use, and containing aromatic polycarboxylic acids as essentialingredients, a need has arisen for an efficient commercial process forproducing the various aromatic polycarboxylic acids. In the past, thepreparation of these acids has been ditficult to achieve on a commercialscale, and commercial utilization of the acids, particularly ofisophtalic acid and terephthalic acid, has been hindered.

While commercial processes for the oxidation of alkylated benzenes tothe corresponding substituted benzoicacids have been known, it has beengenerally recognized that oxidation of any alkyl group on the aromaticnucleus is dilficult to achieve in the presence of an already formedcarboxylic acid group. Thus the art has resorted to variousmultiple-staged operations in order to accomplish this oxidationsuccessfully, and such processes involving conversion of alkylatedbenzoic acids to more readily oxidizable intermediates, such as thesalts or esters thereof are known. Such processes are complex andexpensive, and considerable research effort has been expended in seekinga method for direct oxidation of alkyl substituted benzoic acids topolycarboxylic acids such as isophthalic and terephthalic acid.

It is an object of the present invention to provide an economical andpractical method for oxidizing alkyl substituted aromatic acids in theliquid phase with oxygen containing gases to produce aromaticpolycarboxylic acids. Another object is to provide a method for carryingout such oxidation reactions in the presence of novel,oxidation-resistant solvents to obtain relatively high yields of thedesired polycarboxylic acids. It is a further object of this inventionto provide improvements in the process for the preparation of phthalicacids by the oxidation of alkyl substituted benzoic acids e.g. by

the oxidation of the methyl group of toluic acids, with,

Other objects molecular oxygen in the liquid phase. and advantages ofthe present invention will be' apparent to those skilled in the art fromthe more detailed description which follows.

It has now been discovered that alkyl substituted this invention, analkylated benzoic acid may be converted by oxidation in the liquid phaseat elevated temperatures with an oxygen containing gas to a dibasic d-..has qes dq qy. be. 2 3M 19 sa d i United States PatentO a product of theoxidation reaction, or may comprise the neutral ester of an isomer ofsaid acid or neutral esters of mixtures of isomeric acids. For purposesof simplicity and ease of operation, itis preferred that the solvent bean ester of the polycarboxylic acid produced in the oxidation reactionparticularly in order to facilitate the recovery of the products ofoxidation as will be =further described hereinafter.

The charge storck to the oxidation process comprises an aromaticmonocarboxylic acid possessing at least one oxidizable alkyl group as anadditional ring substituent. The oxidizable group is one which has atleast one hydrogen atom on the carbon atom attached to the aromaticring. The alkyl group need not be restricted in chain length, but forpractical purposes, lower alkyl groups of 1 to 4 carbon atoms arepreferred. The charge stock may comprise a singe compound, or mayconsist of a mixture of alkylated aromatic acids such as mixtures ofisomeric toluic acids obtained, for example, by partial oxidation ofmixed xylenes. The aromatic acids may contain one or more partiallyoxidized alkyl substituents such as acetyl groups, etc. which may beeffectively further oxidized by the method of the invention. The processmay not only be applied to various aromatic acids obtained by oxidationof one alkyl group of a hydrocarbon such as xylene, mesitylene,pseudocumene, durene, cymene and the like, but also to alkylatedpolynuclear aromatic acids such as methyl naphthoic acid and the like.

The ester solvent utilized in the invention is preferably a neutrallower alkyl ester of the aromatic acid to be produced in the oxidationreaction. Thus in the oxidation of toluic acids it is preferred toutilize lower alkyl ester of phthalic acid, which ester is furtherpreferably an ester of the same isomeric acid to be formed in theoxidation. When p-toluic acid is to be oxidized, the preferred ester isdirnethyl terephthalate. Other esters which will serve equally well arefor example, neutral esters of the methyl phthalic acids, neutral estersof tbutyl phthalic acid, trimethyl trimesate, tetramethyl prehnitate,tetramethyl pyromellitate. While the methyl esters are preferred,esters. of other alcohols, for example, aliphatic alcohols of 2 to 8carbon atoms may be employed. The ester solvent may comprise a singlecompound, or mixtures of esters may be utilized. The only requirementfor the solvent is that its melting point lie below the oxidationtemperature, and to this end esters which form eutectic mixtures witheach other may be advantageously utilized to insure liquid phasereaction conditions at the oxidation temperature.

While the proportions of alkylated aromatic acid and aromatic estersolvent are not critical, at least 10% by volume of ester solvent shouldbe employed, and preferably a larger amount for example, 20 to volumepercent is present. The lower concentrations of ester solvent, can beemployed, but the oxidation mixture containing substantial amounts ofaromatic dibasic acid may be difiicult to handle due to precipitation ofthe dicarboxylic acid formed by oxidation. When higher concentrations ofoxidizable charge stock are employed, the conversion need not be carriedto completion, but the oxidation may be interrupted to yield a solutionor slurry of polybasic acid which may be more readily processed forrecovery of the desired polybasic aromatic acid.

or semi-continuous manner. tinuous manner, provision may be made forremoval of The oxidation step is conducted by contacting a mixture ofaromatic diester and alkylated aromatic acid with a free oxygencontaining gas at elevated temperature in an oxidation zone to produce amixture of polybasic aromatic acid and starting ester. Surprisingly, ithas been found that the diester solvent is particularly resistant tooxidation, and may be recovered and recycled to the oxidation zonewithout appreciable loss. The aromatic acids so formed may be separatedfrom the ester solvent or converted to the corresponding esters andrecovered as such.

The oxidation step may be conducted in the presence of a known metalliccatalyst if desired, although in many cases a catalyst may not benecessary, depending-- upon the resistance of the charged aromatic acidto oxidation.

For this purpose, the compounds of metals having molecular weights fromabout 50 to about 210, and especially the heavy metals, may be utilized.Compounds 'ofcobalt, vanadium, manganese, chromium, nickel, lead,cerium, barium and the like, are all effective catalysts in the process,the cobalt compounds being preferred. The metallic catalysts may beutilized singly, or as a mixture of one or more metallic compounds. Themetals may be utilized in the form of their oxides, or hydroxides in theform of oil soluble compounds. Examples of catalysts which give goodresults are cobalt salts of organic acids, for example, salts of lowerfatty acids, e.g. acetates; of higher fatty acids, e.g. caprylates orlinoleats; of cyclic aliphatic acids, e.g. naphthenates; and of aromaticacids, e.g. benzoates or toluates. 'Concentrations in the range of about0.0010.5% by weight of metal based on total weight of the substitutedaromatic acid to be oxidized are generally effective.

As has been mentioned, the mixture of alkyl aromatic acid and diestersolvent is contacted inthe liquid phase at elevated temperature withoxygen or an oxygen containing gas, for example with air or air whichhas been enriched by admixture with oxygen. The oxidation is conductedat temperatures of from about 150 "to about 300 C., depending upon theresistance of the particular feedstock to oxidation, the lowertemperatures being preferred to avoid decomposition of the desireddibasic acids which leads to reduced yields of impure products.

It will be recognized by those skilled in the art that time andtemperature are interrelated variables, the oxidation at more elevatedtemperatures generally requiring less time to achieve a yield comparablewith that obtained over a longer reaction period at lower temperatures.Generally, a reaction time'of from 2 to 24 hours will be suificient, andthe choice of reaction time will be dictated not only by the rate ofoxidation, but also by the'degree of conversion desired. The oxidationis advantageously interrupted where the product of reaction is insolubleor partially insoluble in the reaction medium at reaction temperature toavoid formation of thick slurries of dibasic acid which are difiicult toagitate effectively and thus cause inefficient utilization of oxygen inthe later stages of the oxidation.

The oxidation can be conducted at atmospheric pressure, or underelevated pressures for example, up to about 100 atmospheres. -The use ofelevated pressure is preferred and results in a faster rate ofoxidation, thus reducing the time during which the reaction mixture mustbe kept at elevated temperatures.- It will be realized that reactiontemperature and pressure are interrelated variables, the preferredtemperature being lower within the range indicated when superatmosphericpressure is employed.

-'The oxidation can be conducted in batch, continuous When operated in acondibasic acid formed during the reaction by cooling and filtering thereactor effluent continuously and for addition of alkyl benzoic acid tothe recovered diester solvent which may be recycled to the reactor. Whenoperating in a continuous manner, it may be preferable to operatetheo'x'idation reaction so as to obtain only partial conversion of thealkyl benzoic acid feedstock in order to facilitate separation of thedibasic acid and recycle of the ester solvent.

The separation and extraction of the dibasic acids from the oxidationreaction effluent may be carried out in any one'of anumber of ways.Generally the reactor effluent will contain a mixture of someunconverted alkyl benzoic acid feedstock, and the diester solvent, aswell as the desired dicarboxylic acid. The reaction mixture may betreated with dilute alkali to remove all acidic constituents,'and*theacids'recovered by treatment of the alkaline solution withstrong mineralacids. The mixture of organic acids which is thus recovered may befurther treated, for example, by extraction with hot water, to achieve aseparation of the mono basic acids from the less soluble dibasic acids.

Alternatively, the reaction mixture may be esterified by conventionalprocedures to produce a mixture of esters which may be separated byfractional distillation. Where the dibasic acid formed corresponds tothe acid component of the'diester solvent utilized in the oxidation, theesterified mixture may be fractionated to remove esters of mono basicacids, a portion of the residual diester being taken as product, and theremainder recycled directly to the oxidation reactor as solvent forsubsequent oxidations. Other methods of recovery of the desired acidwill occur to one skilled in the art, for example, by simple filtration'where the desired acid is insoluble or partially insoluble in thecooled oxidation efiluent, or 'by saponification of the total reactorefiiuent, followed by recovery of a mixture of acids and separation inconventional manner. The latter method is particularly effective Wherethe dibasic acid formed by oxidation is the same as theacid component ofthe ester solvent utilized, no further separation of the dibasic acidbeing required.

The following examples are presented as an illustration of the practiceof the process of the invention.

'EXAMPLE 1 A mixture of 3.4 grams m-toluic acid and 30.6 gramsdimethylisophthalate with 0.2 wt. percent cobaltous acetate and 0.9wt."percent yellow lead oxide (PbO) was oxidized in aglass linerinstalled in a 500 ml. rocker bomb. Oxygen (0.368 mole) was pressuredin. The bomb was rocked and the temperature was raised stepwise over a 3hour period to 200 C., held at 200 C. for 30 minutes, at 225 C.for 30minutes and at 250 C. for minutes. Maximum pressure was 515 psi. at 250C. The product was saponified by refluxing with 2.5 g.'KOH in ml.'ethanol; water (100 ml.) was added and alcohol distilled to a boilingpoint of 99 C. The residue was acidified with 96% sulfuric acid,digested 20 minutes on the steam bath, filtered and washed with water.The solid acid and soluble organic acid (recovered from the filtrate byether extraction) were combined, dried under vacuum (28.1 g.) andanalyzed by a chromatographic technique similar to that of Marvel andRands (J. Am. Chem. Soc., 1950, 72, 264-2). This indicat ed that 88.1%of the m-toluic acidhad been oxidized;

92.9 of the organic acids had been accounted for.

EXAMPLE 2 500 p.s.i."oxyg"en '(0.106'mole) for six hours at 250 C.

The bomb was cooled, vented and-repressured with oxygen (0.095 mole).-The oxidation was continued for six hours at250 -C. The product wassaponified by re fluxing with 200' g. 10% NaOH (aq.). Methanol wasdistilled 'to-a boiling point of 99 C. The residue was ass as? filteredthrough fritted glass and the filtrate acidified with sulfuric acid. Thesuspension was digested on a steam bath. The precipitated acid wasfiltered and washed with water. This solid acid and the soluble organicacid (recovered from the filtrate by ether extraction) were combined anddried under high vacuum. Analysis revealed that 79.7% of the m-toluicacid had been oxidized to isophthalic acid. A material balance of 91.4%was realized on the organic acids.

EXAMPLE 3 A mixture of 3.0 g. m-toluic acid and 12.0 g. dimethylisophthalate was oxidized without catalyst in a glass liner installed ina 200 ml. rocker bomb. Two five hour oxidation periods (0.107 and 0.100mole respectively) at 250 C. and 500 p.s.i. gave a product that onsaponification (as in Example 2) revealed that 82.3% of the m-toluicacid had been oxidized to isophthalic acid. The organic acid materialbalance was 95.0%.

EXAMPLE 4 A series of single stage oxidations was carried out underidentical conditions. Each run was made in a glass liner installed inthe 200 ml. rocker bomb. The table lists the conditions employed and theresults obtained. The products were worked up as in Example 2. It can beseen that both mand p-toluic acids are more easily oxidized than themethyl toluates. It is also apparent from the table that methylm-toluate has been more easily oxidized than methyl p-toluate contraryto what would have been expected had no solvent been employed.

Table.0xidations in dimethyl isophthalate (DMI) Toluic AcidMethyltoluate oxidant (A') metaparametapara- Mole percent in DMI:

Toluic Acid 26. 3 26. 3 Methy 26. 3 26. 3 Mole Ratio, 022 A 4. 66 4. 664. 62 4. 66 Time at 250 0. (hrs.) 5% 5% 5% 5% Wt. percent Cobaltousn-caprylate 0.2 0.2 0.2 0.2 Percent of Toluic acid (or ester) Oxidized87. 2 92. 9 65. 9 64. 6 Material Balance on organic acid content 96. 499. 7 97. 8

Dimethyl isophthalate was employed in the above experiments because ofits low melting point (64-65% C.). However, dimethylterephthalate (M.P.140 C.), dimethyl o-phthalate (liquid), trimethyl hemimellitate (M.P.100 C.), trimethyl trimellitate (liquid), trimethyl trimesate (M.P. 143C.), tetramethyl prehnitate (M.P. IDS-109 C.), tetramethyl pyromellitate(M.P. 138 C.), or eutectic mixtures of similar solvents might also beemployed for the oxidation of toluic acids or more highly alkylatedbenzoic acids. The only requirement for a solvent would be that itsmelting point lies below the oxidation temperature.

The above examples are illustrative only and are not intended to limitthe invention in any way. Numerous modifications of the invention willoccur to those skilled in the art. The reaction vessel may be in theform of a stirred autoclave or in any other suitable form, such as atower or a horizontal tubular reactionunit. The process may be operatedbatchwise or continuously, and in continuous operation the reactants maybe passed through the oxidizer concurrently or countercurrently. Anysuitable method may be employed for obtaining adequate contact betweenthe oxidizing gas and the liquid reaction medium. Since numerous methodsfor practicing the invention will occur to those skilled in the art, itwill be understood that I do not limit myself except as set forth in thefollowing claims.

What I claim is:

1. A process for the preparation of an aromatic dicarboxylic acid whichcomprises oxidizing an aromatic monocarboxylic acid having at least oneoxidizable alkyl substituent on the aromatic ring in a solventcomprising a neutral lower alkyl ester of a polycarboxylic aromatic acidin the liquid phase with a gas containing molecular oxygen at atemperature of from about 150 C. to about 300 C. and at a pressure offrom about 1 to 100 atmospheres.

2. The process of claim 1 wherein the oxidation is carried out in thepresence of a metallic oxidation catalyst.

3. A process for the preparation of a benzene dicarboxylic acid whichcomprises oxidizing a benzene monocarboxylic acid having at least oneoxidizable alkyl substituent on the benzene ring in a solvent comprisinga neutral lower alkyl ester of a benzene polycarboxylic acid in theliquid phase With a gas containing molecular oxygen at a temperature offrom about 150 C. to about 300 C. and at a pressure from about 1 to 100atmospheres, said mixture containing from about 10% to about by volumeof benzene monocarboxylic acid.

4. The process of claim 3 wherein the oxidation is carried out in thepresence of from about 0.001 to about 0.5% by weight of a metallicoxidation catalyst based on the weight of the benzene monocarboxylicacid.

5. The process of claim 4 wherein the oxidation is carried out in thepresence of a cobalt salt as oxidation catalyst.

6. A process for the preparation of a phthalic acid which comprisesoxidizing a toluic acid in the liquid phase with a gas containingmolecular oxygen at a temperature of from about 150 C. to about 300 C.and a pressure of from about 1 to atmospheres in the presence of atleast 10% by volume of a neutral lower alkyl ester of phthalic acid.

7. The process of claim 6 wherein the neutral lower alkyl ester ofphthalic acid is the dimethyl ester.

8. The process of claim 7 wherein the toluic acid is para-toluic acid.

9. The process of claim 7 wherein the toluic acid is meta-toluic acid.

10. In a process for the preparation of terephthalic acid by the liquidphase oxidation of para-toluic acid with molecular oxygen in thepresence of a metallic oxidation catalyst at a temperature of from aboutC. to about 300 C. and at a pressure of from about 1 to 100 atmospheres,the improvement which comprises conducting the oxidation in the presenceof from about 10 to 90% by volume of the reaction mixture of dimethylterephthalate as solvent.

11. In a process for the preparation of isophthalic acid by the liquidphase oxidation of meta-toluic acid with molecular oxygen in thepresence of a metallic oxidation catalyst at a temperature of from about150 C. to about 300 C. and at a pressure of from about 1 to 100atmospheres, the improvement which comprises conducting the oxidation inthe presence of from about 10 to about 90% by volume of the reactionmixture of dimethyl isophthalate as solvent.

Toland Nov. 21, 1950 Toland Nov. 21, 1950

1. A PROCESS FOR THE PREPARATION OF AN AROMATIC DICARBOXYLIC ACID WHICHCOMPRISES OXIDIZING AN AROMATIC MONOCARBOXYLIC ACID HAVING AT LEAST ONEOXIDIZABLE ALKYL SIBSTITUENT ON THE AROMATIC RING IN A SOLVENTCOMPRISING A NEUTRAL LOWER ALKYL ESTER OF A POLYCARBOXYL AROMATIC ACIDIN THE LIQUID PHASE WITH A GAS CONTAINING MOLECULAR OXYGEN AT ATEMPERATURE OF FROM ABOUT 150*C. TO ABOUT 300*C. AND AT A TEMPERATURE OFFROM ABOUT 1 TO 100 ATMOSPHERES.